1. Field of the Invention
The present invention relates to power circuit control systems requiring galvanic isolation between the control and the power circuit. Such systems are generally used for controlling loads supplied by the A.C. power system. The galvanic isolation essentially has the function of protecting the control circuit and its user.
2. Discussion of the Related Art
FIG. 1 very schematically shows a first conventional example of a device for galvanic isolation between a control circuit and a power circuit. This device is a transformer T forming an isolation barrier IB and having a primary winding T1 connected to two output terminals of a control circuit 1 and a secondary winding T2 that controls a power switch K. In the example of FIG. 1, switch K is a thyristor connected between two terminals 2 and 3 of the power circuit, not shown. The (cathode) gate of thyristor K is connected to a first terminal of winding T2, the other terminal of which is connected to terminal 3 generally representing a reference voltage (for example, the ground). On the primary side, control circuit 1 is generally supplied by a low-voltage source (not shown). Either low-frequency control transformers (up to a few tens of kilohertz), or transformers excited by a synchronous pulse upon each halfwave of an A.C. supply voltage on the secondary side are provided.
A disadvantage of control systems of the type illustrated in FIG. 1 is that a discrete transformer is required, which is thus bulky and expensive.
FIG. 2 shows a second known example of a galvanic isolation control system. In the example of FIG. 2, the crossing of isolation barrier IB is optical. An optocoupler OP, the excitation diode D of which receives a control signal from a circuit 4 (CTRL) and the phototransistor TO of which provides a control signal to a circuit 5 (PWCTRL) of control of a switch K is for example used. As previously, switch K is connected across two terminals 2 and 3 downstream of the isolation barrier. In the example of FIG. 2, said switch again is a thyristor having its gate connected to control circuit 5.
Even though the optocoupler may in some cases be integrated, the use of control electronics (circuit 5) downstream of the isolation barrier most often is a disadvantage.
Other galvanic isolation barriers are known. For example, the simplest is formed of capacitors placed on each conductor to be isolated. The capacitors must then hold high voltages and are thus bulky and expensive. Further, as in the case of FIG. 2, they require electronic circuitry to control the thyristor or the triac that constitutes the switch downstream of the isolation barrier.